The Fischer-Tropsch condensation process is a reaction which converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g. 125 to 300° C., preferably 175 to 250° C.) and/or pressures (e.g. 5 to 100 bar, preferably 12 to 50 bar).
The Fischer-Tropsch process can be used to prepare a range of hydrocarbon fuels, including LPG, naphtha, kerosene and gas oil fractions. Of these, the gas oils have been used in middle distillate fuel compositions such as in particular automotive diesel fuels, typically in blends with petroleum derived gas oils. The heavier fractions can yield, following hydroprocessing and vacuum distillation, a series of base oils having different distillation properties and viscosities, which are useful as lubricating base oil stocks. These base oils have a wide range of uses, including as lubricants, as dielectric fluids (for example electrical oils or transformer oils), as hydraulic fluids (for example in shock absorbers) and as process oils for instance in elastomer production.
WO-A-02070627 and WO-A-02070629, for example, describe processes for preparing iso-paraffinic base oils from a wax made by a Fischer-Tropsch process. Such Fischer-Tropsch derived base oils tend to have excellent low temperature properties, for example low pour points, and are also attractive because of the relatively simple process used to make them as compared to similar oils prepared from mineral crude sources.
The products of such a process include, inter alia, a gas oil and a base oil stream having a nominal kinematic viscosity at 100° C. (VK 100) of about 4 centistokes, which is suitable for use in lubricant formulations.
They also include a light base oil, of VK 100 from about 2 to 3 centistokes, which has boiling points between the final boiling point of the gas oil and the initial boiling point of the 4 centistoke base oil. The boiling range of this intermediate product is determined by those of the gas oil and the 4 centistoke base oil, and the initial boiling point of the 4 centistoke oil is fixed due to the viscosity and volatility specifications with which it has to comply. This leads to a relatively low degree of control over the boiling range, and hence over related properties such as viscosity and pour point, for the 3 centistoke light base oil. This in turn can restrict the potential applications of the light oil, as it can be difficult to formulate to a desired specification. Its viscosity and pour point may for example be inappropriately high for the oil to be of use as an electrical oil, yet its viscosity will also be too low for it to be of use as a lubricant oil.
Thus it would be desirable to be able to exercise greater control over the properties of the light base oil cut, so as to extend its potential applications. Depending on its intended use, a base oil often needs to comply with strict requirements regarding its viscosity and viscosity index, its flash point, its distillation properties and its flow properties (in particular its low temperature performance). A high flash point is particularly important for base oil formulations which are intended for use as electrical oils, especially when used in a high temperature environment or in situations involving elevated peak temperatures. Thus if a light base oil is to be of use in such a context, not only its viscosity and its pour point, but also its flash point, must be carefully controlled so as to meet applicable specifications.
It has now surprisingly been found that when formulating a Fischer-Tropsch derived light base oil, improved properties can be achieved without an undesirably high reduction in flash point, by the addition of a Fischer-Tropsch derived gas oil. Thus a Fischer-Tropsch derived gas oil may be used to tune the properties of a light base oil. This in turn can increase the versatility of the light oil, and in particular can render the resultant blend suitable for use as an electrical oil.